1. Field of the Invention
The present invention relates generally to control circuitry for power supplies and, in particular, to control circuitry for use on the load side of a switch-mode power supply.
2. Description of Related Art
Switching mode power supplies are frequently used to power small appliances such as portable computers and the like where electrical isolation from an AC power source is needed. One advantage of switching mode supplies is that they can be compact in size and yet deliver considerable power. FIG. 1 is a simplified block diagram of a typical prior art switching mode power supply. A rectifier/filter circuit 10 is connected to an AC power source that is typically 110 V 60 Hz for domestic application and 85-265 V, 50/60 Hz for international applications. The output of the rectifier/filter 10 is connected to the primary side of an isolation transformer 12. A Line-Side Controller 14 includes an active switch, such a power transistor 16, which operates to periodically interrupt the connection between the transformer primary and the output of the rectifier/filter circuit 10 so as to introduce an AC component. The switching frequency is relatively high so that transformer 12 can be made relatively small. Details of an exemplary Line-Side Controller 14 are disclosed in U.S. Pat. No. 6,233,165, the contents of which are fully incorporated herein by reference.
The output of the secondary winding of transformer 12 is rectified by diode 18 and filtered by capacitors 20 and 24 and choke 22 to produce a DC output voltage Vout+. A Load-Side Controller 26 is included to provide various functions. Among other things, Load-Side Controller 26 provides a control input to the Line-Side Controller 14 by way of optical coupler 28 so as regulate the magnitude of Vout+using pulse width modulation. Load-side Controller 26 also typically provides output short circuit protection and output current limiting functions. Preferably, the Load-Side Controller 26 is powered by the output Vout of the power supply or output V+and does not require the use of an auxiliary power supply.
FIG. 2 is a schematic diagram of an exemplary prior art Load-Side Controller 26 for use in the FIG. 1 switch mode power supply. The FIG. 2 controller is fabricated from discrete components including Zener diode Z1 which is connected to V+ by way of a resistor Rz. The output voltage Vout+, which is related to voltage V+, is equal to the sum of the forward voltage dropped across optical coupler diode 28A, diode D1 and the Zener voltage of Z1. The Zener voltage may be, for example, +4.7 volts. Thus, voltage V+ is approximately +6 volts. If voltage V+ should drop below the target voltage, the current through optical coupler diode 28A will drop, with the Line-Side Controller 14 (FIG. 1) being implemented so as to respond by increasing the output voltage using pulse width modulation. The magnitude of the output voltage can be changed somewhat by changing the value of resistor Rz thereby changing the operating point of the relatively soft knew of Zener diode Z1.
The FIG. 2 load-side controller 26 further includes an NPN transistor Q1 that performs a current limit function. The load current flows through resistors RB and RC, with a fraction of the voltage developed across RB being applied to the base-emitter junction of transistor Q1. At normal output currents, the voltage across RB is not great enough to turn on Q1. However, at greater currents, transistor Q1 will turn on thereby drawing current through resistor RE and the optical coupler diode 28A. The Line-Side Controller 14 (FIG. 1) will respond by reducing the output voltage V+ thereby limiting the output current.
The base-emitter voltage needed to turn transistor Q1 on will vary from transistor to transistor. Thus, it will typically be necessary to manually adjust the value of RB to achieve the desired current limit point. In addition, the base-emitter voltage has a fairly strong negative temperature coefficient. Resistor RT is thermistor device having a positive temperature coefficient resistance that tends to offset the negative temperature coefficient of the base-emitter voltage of transistor Q1. In the event that the output of the supply becomes shorted, that is V+ (or Vout+) is shorted to Voutxe2x88x92, the current limit circuit will continue to operate. No auxiliary power supply is needed to keep the current limit circuit functioning under these conditions.
Resistor RB is selected to produce a voltage greater than that necessary to turn transistor Q1 on, typically the voltage across RB being around 800 millivolts. Resistor RA, RT and RD function to apply only a fraction of this voltage to the base-emitter junction of Q1. Resistor RC is added to produce another approximately 400 millivolts at current limit. Accordingly, when V+ and Voutxe2x88x92 are shorted together, a total voltage of 1280 millivolts is dropped across resistors RB and RC for powering the current limit circuitry. That is sufficient to power the current limit circuitry which needs about 1300 millivolts to operate, with that being the sum of the voltage drop across resistor RE, coupler diode 28A and the collector-emitter saturation voltage of transistor Q1.
In order to improve the operation of the FIG. 2 controller, some prior art load-side controllers utilize a commercial integrated circuit programmable shunt regulator in lieu of Zener diode Z1. These shunt regulators, such as regulator sold by Fairchild Semiconductor under the designation TL431 include a band-gap circuit that produces a reference voltage and an error amplifier which compares the reference voltage with the output voltage Vout+, or some fraction of the output voltage set by a resistor divider, so as to provide an adjustable regulated output voltage.
The FIG. 2 controller is inherently imprecise in terms of both voltage regulation and current limit set point. Further, a discrete implementation significantly increases the cost of manufacturing a power supply using the FIG. 2 controller, with low cost being a important factor in this type of power supply.
FIG. 3 shows another prior art Load-Side Controller 26, the primary components of which are implemented in integrated circuit form. Rectification is carried out by a Schottky diode D3 connected to the Vxe2x88x92 output of transformer 12 (FIG. 1) which replaces diode 18. The FIG. 3 controller 26 includes an auxiliary power supply AS having an input terminal connected to Vxe2x88x92. The auxiliary supply AS, which typically includes at least a rectifier diode and filter capacitor (not depicted), has an output connected to power input terminal VCC of the integrated circuit I1 of the FIG. 3 controller.
The integrated circuit I1 includes a band-gap regulator circuit BR that produces a band-gap reference voltage of +2.5 volts. Circuit BR can be trimmed to vary the magnitude of the reference voltage. The reference voltage output of the circuit BR is buffered by a unity gain configured amplifier A1, with the output of A1 being coupled to an inverting input of another amplifier A3 by way of resistor RH. A frequency compensation capacitor Cd is connected between terminal Comp, connected to the inverting input of amplifier A3, and terminal Opto.
The non-inverting input of amplifier A3 is connected a terminal V Sense of the integrated circuit I1. A selected fraction of the output voltage V+ is supplied to terminal V Sense by way of a resistive divider network comprising discrete resistors RI and RJ connected between V+ and Voutxe2x88x92. The output of amplifier A3 is connected to terminal Opto by way of a diode D1. The anode of an optical coupler diode 28A is connected to terminal Opto so that current is supplied to diode 28A by way of diode D1 when the amplifier A3 output is positive.
An external discrete resistor RE is connected between the cathode of diode 28A and a terminal GND of the integrated circuit, with a second external resistor RG being connected between terminal Opto and terminal GND. Voltage regulation is achieved by comparing the selected fraction of the output voltage at terminal V Sense with the reference voltage produced by band-gap circuit BR and modulating the current through optical coupler diode 28A in response to the comparison. As previously noted, the Line-Side Controller 14 (FIG. 1) will adjust the magnitude of the output voltage V+ (or Vout+) by way of pulse width modulation in response to the output of the optical coupler 28. Increased current flow through coupler diode 28A will cause the output voltage V+ to decrease whereas decreased current flow will cause the output voltage to increase.
The FIG. 3 Load-Side Controller further provides a current limit function. A second reference circuit RF is used to generate a 200 millivolt reference voltage which is buffered by an unity gain configured amplifier A2. The output of the amplifier is coupled to the inverting input of an amplifier A4 by way of a resistor RK. The non-inverting input of amplifier A4 is connected to terminal I Sense that, in turn, is coupled to Voutxe2x88x92. The output of amplifier A4 is connected to the terminal Opto by way of a diode D2. A small value external current sense resistor RF is connected between terminals I Sense and GND, with the I Sense terminal also being connected to Voutxe2x88x92. Thus, all of the load current will flow through resistor RF.
The inverting input of amplifier A4 receives the buffered 200 millivolt reference voltage produced by reference circuit RF. The voltage applied to the non-inverting input of the amplifier is equal to the voltage drop across current sense resistor RF. Thus, when the voltage drop across resistor RF reaches 200 millivolts, the current limit point, the output of amplifier A4 will increase thereby causing current to flow through optical coupler diode 28A to increase. The Line-Side Controller 14 (FIG. 1) will respond by reducing the magnitude of the regulated output voltage Vout+ thereby limiting the output current. Diodes D1 and D2 operate to isolate the outputs of amplifiers A3 and A4 when the amplifier outputs are low. Thus, either the voltage regulating amplifier A3 or the current limiting amplifier A4 can independently cause current to flow through the optical coupler diode 28A.
The FIG. 3 Load-Side Controller is superior to the FIG. 2 controller in terms of voltage regulating accuracy and current limit accuracy. Further, the FIG. 3 controller is less expensive to implement in that most of the components are part of integrated circuit I1. However, the FIG. 3 circuit requires the use of auxiliary power supply AS for powering at least the current limit circuitry in the event the output is shorted. As previously noted, it is desirable to avoid an auxiliary power supply in order to reduce costs.